I did some quick math and I don't see why we couldn't make Butler, Durant and Horford work. We'd have to gut our bench but that should be a consideration when building around someone like Durant.
Once we renounce all of our RFAs and decline the options on JJ and Amir we'll have $60,028,372 in cap space.
We trade KO, Rozier and James Young, along with whatever picks necessary wants for Butler. The deal doesn't matter as long as those salaries go out, or similar.
At that point we'd be $500 shy of 2 maximum contracts. I can't imagine that would be a sticking point for either Horford or Durant if they want to sign here.
The problem isn't making the $ work so much as keeping our defensive core in tact while acquiring Butler. The Bulls will want Crowder and/or AB in the deal.